1. FIELD OF THE INVENTION
This invention is directed to an improvement in the electrical system of a plasma gas flame spraying process. More particularly, this invention is directed to means for insuring that when a secondary gas is passed through an electric arc established in a flame spray gun, which causes an increase in the voltage in the current of said circuit containing the electric arc, the current is suitably adjusted. This invention is also directed to means for insuring that ignition of the electric arc is established by the pressing of a single switch. The invention is also directed to a closed loop system of determining the voltage changes in a circuit containing an electric arc for use in plasma flame spraying and for compensatingly regulating the current at a fixed plasma gas flow-through rate and composition to maintain a desired balance between current and secondary plasma gas flow. This invention is directed to various automatic means for insuring a longer electrode and nozzle life and for insuring against excessive constriction of the confined electric arc within the nozzle of the flame spraying gun or parts associated therewith.
2. DISCUSSION OF THE PRIOR ART
Plasma flame spraying is a particular method whereby at least one gas is caused by virtue of its passage through an electric arc to be put into a plasma state. This plasma state corresponds to a higher energy state than the gaseous state. At such higher energy state, it has been found the gas assumes properties whereby it is an excellent heating medium. It has been disclosed, for instance, in U.S. Pat. No. 2,960,594, that extremely high temperatures on the order of 8,500.degree. F. and upwards can be provided by passing a mixture of gases through a nozzle containing an electric arc. The arc is established between two oppositely polarized electrodes employing a current generally in the range of 155-1,000 amps. The gas can be heated to such an extent that powder fed at the nozzle of the gun can be so melted or heat softened that it can be sprayed onto a relatively cool workpiece. The high energy plasma state of the gas causes the particles to assume an elevated temperature state whereby they readily adhere to the workpiece of entirely different temperature. Numerous gases for use in plasma gas flame spraying can be used. These include in particular nitrogen, which has been found to provide an excellent primary gas.
In the development of the flame spraying technology as above described, it was found that additional gases, denominated secondary gases, provide extremely desirable properties. Thus, a minor amount of hydrogen added to a nitrogen or argon stream vastly improves the temperature of the plasma gas. Other typical secondary gases comprise: helium added to A or N.sub.2, argon added to N.sub.2, and nitrogen added to argon. However, the introduction of such hydrogen across the electric arc established in the nozzle of the gun increases the voltage through the circuit containing the electric arc at a phenomenal rate. Such voltage increase, if left uncompensated, causes a substantial diminishment of the circuit.
The flame spraying nozzles are so constructed that the arc is caused to be positioned over a relatively wide area within the nozzle. However, problems can be caused during start-up before the plasma gun has reached its optimum temperature, gas flow composition and flow rate, and arc distribution. Specifically, it has been found that a sheath of plasma forming gas around the arc tends to cool the outer periphery of the arc. This, in turn, reduces the amount of ionization of the plasma forming gases. Such reduction causes an increase in the electrical resistance of the outer periphery. This, in turn, causes more current to flow through the path of lesser resistance at the core of the arc, thus constricting the arc and increasing its temperature. The constriction of the arc is progressive, being greater at the inner end of the nozzle. By adjusting the proportion of the flow of gas to the flow of current, the arc can be made to spread gradually as it extends down the nozzle bore. This enables the arc to be spread in contact with any desired point in the bore. Indeed, an arc can even be established out of the nozzle toward the workpiece by suitable regulation of the flow of gas and the current to the arc.
It can be seen, therefore, that problems can arise due to excessive constriction of the arc. Specifically, if the arc constricts too much and becomes concentrated, damage can be done to the nozzle assembly. Because the temperatures employed are extremely high and the voltage within the circuit of the arc is extremely high, such damage can occur in a relatively short period of time. It has been found that even experienced operators of manual plasma gas flaming units cannot always adjust the gas flow against the current or resistance in the circuit of the arc such as to avoid the problem mentioned above. Hence, it has become highly desirable to provide a means whereby establishment of a desired broad arc can be accomplished automatically without any problems developing which could cause constriction of the arc and damage to the nozzle assembly.
It was initially believed that a suitable start-up procedure involved adjustment of the arc current to an arc current of 300 amperes. A secondary gas which would normally cause an increase in the voltage (a decrease in the current) could then be admitted through a needle valve. Voltage could be regulated by allowing the secondary gas to enter and be raised up until full flow, insuring that the amperage did not drop more than 25 amps for every five point increase in secondary gas flow. The amperage was not allowed to drop below 250 amps. However, this required some manual dexterity in that the arc current had to be adjusted with one hand while the secondary gas flow increase (or decrease) was adjusted with the other. Such a method required a certain quantum of skill not always possessed by those skilled in the flame spraying process itself, especially those operators somewhat unfamiliar with plasma gas flame spraying processes.
Hence, it has become highly desirable to provide an automatic means which will adjust the current increase and secondary gas flow rate such that optimum conditions are maintained in the plasma gas flame spraying nozzle. Such optimum conditions include a balance of voltage and current against secondary gas flow such that constriction of the electric arc does not occur. It has also become desirable to provide a means for accomplishing the same through use of timers independently associated with a secondary gas flow regulator and a regulator in electrical association with the arc current control, whereby an increase in both is responsive to time and the full flow of the secondary gas is established within a narrow time window based upon establishment of the final current through the electric arc.
Another problem has developed in the development of suitable automatic system for electrical control of the ignition and shut-down procedure. For some mysterious reason, occasionally the energization of a system would not initiate the electric arc. Since an automatic system is dependent upon the establishment of such an arc, problems resulted. Whereas, in manual operation the operator had only to throw a switch a second or third time to provide initiation of the electric arc, the automatic system is necessarily premised upon establisment of an electrical arc when a start-up switch is thrown. Hence, it has become desirable to provide a means for insuring the establishment of an arc during start-up.
It has become desirable also to provide a fully complete closed loop control automatic electrical system for a plasma gun, which system is capable of regulating an increase or decrease in the arc current or the secondary gas flow rate in response to electrical considerations determined within the nozzle itself.